arXiv subjects

Cheongho Han

Publications and source records attributed to Cheongho Han.

At least 181 records · Page 10Linked to original sources

arXiv2013

Reanalysis of the Gravitational Microlensing Event MACHO-97-BLG-41 based on Combined Data

MACHO-97-BLG-41 is a gravitational microlensing event produced by a lens composed of multiple masses detected by the first-generation lensing experiment. For the event, there exist two different interpretations of the lens from independent analyses based on two different data sets: one interpreted the event as produced by a circumbinary planetary system while the other explained the light curve with only a binary system by introducing orbital motion of the lens. According to the former interpretation, the lens would be not only the first planet detected via microlensing but also the first circumbinary planet ever detected. To resolve the issue using state-of-the-art analysis methods, we reanalyze the event based on the combined data used separately by the previous analyses. By considering various higher-order effects, we find that the orbiting binary-lens model provides a better fit than the circumbinary planet model with Δχ^2 ~166. The result signifies the importance of even and dense coverage of lensing light curves in the interpretation of events.

astro-ph.SR↗

arXiv2013

OGLE-2011-BLG-0417: A Radial Velocity Testbed for Microlensing

Microlensing experiments are returning increasingly detailed information about the planetary and binary systems that are being detected, far beyond what was originally expected. In several cases the lens mass and distance are measured, and a few very special cases have yielded complete 8-parameter Kepler solutions, i.e., the masses of both components, five Kepler invariants and the phase. We identify one such case that is suitable for a precision test that could be carried out by comparing Doppler (RV) measurements with the predictions from the microlensing solution. The lens primary is reasonably bright (I=16.3, V=18.0) and is expected to have a relatively large RV semi-amplitude K ~ 6.35 km/s).

astro-ph.EP↗

arXiv2010

Correlations Between Planetary Microlensing Parameters

Characterization of microlensing planets requires modeling of observed light curves including many parameters. Studying the dependency of the pattern of light curves on the lensing parameters and the correlations between the parameters is important to understand how the uncertainties of the planetary parameters are propagated from other parameters. In this paper, we show that despite the apparent complexity of the pattern of light curves of planetary lensing events, the correlations between the lensing parameters can be understood by studying how the parameters affect the characteristics of lensing light curves such as the height and width, the caustic-crossing time scale, and the location and duration of planetary perturbations. Based on analytic arguments about the dependency of light curve features on the parameters, we obtain the correlations for the two representative cases of planetary events. We also demonstrate the applicability of the correlations to general planetary events by actually obtaining the correlations from modelings of light curves produced by simulations.

astro-ph.SR↗

arXiv2009

Characterization of Stellar Spots in Next-Generation Microlensing Surveys

One of the important microlensing applications to stellar atmospheres is the study of spots on stellar surface provided by the high resolution of caustic-crossing binary-lens events. In this paper, we investigate the characteristics of spot-induced perturbations in microlensing light curves and explain the physical background of the characteristics. We explore the variation of the spot-induced perturbations depending on various parameters characterizing the spot and investigate how well these parameters can be retrieved from observations in high-cadence future lensing surveys. From this, we find that although it would not be easy to precisely constrain the shape and the surface brightness contrast, the size and location of the spot on the stellar surface can be fairly well constrained from the analysis of lensing light curves.

astro-ph.SR↗

arXiv2009

The Importance of Binary Gravitational Microlensing Events Through High-Magnification Channel

We estimate the detection efficiency of binary gravitational lensing events through the channel of high-magnification events. From this estimation, we find that binaries in the separations ranges of 0.1 < s < 10, 0.2 < s < 5, and 0.3 < s < 3 can be detected with ~ 100% efficiency for events with magnifications higher than A=100, 50, and 10, respectively, where s represents the projected separation between the lens components normalized by the Einstein radius. We also find that the range of high efficiency covers nearly the whole mass-ratio range of stellar companions. Due to the high efficiency in wide ranges of parameter space, we point out that majority of binary-lens events will be detected through the high-magnification channel in lensing surveys that focus on high-magnification events for efficient detections of microlensing planets. In addition to the high efficiency, the simplicity of the efficiency estimation makes the sample of these binaries useful in the statistical studies of the distributions of binary companions as functions of mass ratio and separation. We also discuss other importance of these events.

astro-ph.SR↗

arXiv2009

Characterization of Microlensing Planets with Moderately Wide Separations

In future high-cadence microlensing surveys, planets can be detected through a new channel of an independent event produced by the planet itself. The two populations of planets to be detected through this channel are wide-separation planets and free-floating planets. Although they appear as similar short time-scale events, the two populations of planets are widely different in nature and thus distinguishing them is important. In this paper, we investigate the lensing properties of events produced by planets with moderately wide separations from host stars. We find that the lensing behavior of these events is well described by the Chang-Refsdal lensing and the shear caused by the primary not only produces a caustic but also makes the magnification contour elongated along the primary-planet axis. The elongated magnification contour implies that the light curves of these planetary events are generally asymmetric and thus the asymmetry can be used to distinguish the events from those produced by free-floating planets. The asymmetry can be noticed from the overall shape of the light curve and thus can hardly be missed unlike the very short-duration central perturbation caused by the caustic. In addition, the asymmetry occurs regardless of the event magnification and thus the bound nature of the planet can be identified for majority of these events. The close approximation of the lensing light curve to that of the Chang-Refsdal lensing implies that the analysis of the light curve yields only the information about the projected separation between the host star and the planet.

astro-ph.EP↗

arXiv2009

The Demographics of Extrasolar Planets Beyond the Snow Line with Ground-based Microlensing Surveys

In the currently-favored paradigm of planet formation, the location of the snow line in the protoplanetary disk plays a crucial role. Determining the demographics of planets beyond the snow line of stars of various masses is thus essential for testing this model. Microlensing is sensitive to planets that are generally inaccessible to other methods, and in particular is most sensitive to cool planets at or beyond the snow line, including very low-mass (i.e. terrestrial) planets. Hence, microlensing is uniquely suited and so essential for a comprehensive study of this region. Microlensing is also sensitive to planets orbiting low-mass stars, free-floating planets, planets in the Galactic bulge and disk, and even planets in external galaxies. These planets can also provide critical constraints on models of planet formation. Although microlensing searches have so far detected only a handful of planets, these have already changed our understanding of planet formation beyond the snow line. Next generation microlensing surveys, which would be sensitive to tens of "cold Earths" in this region, are well advanced in design conception and are starting initial practical implementation.

astro-ph.EP↗

arXiv2008

Distinguishing between Planetary and Binary Interpretations of Microlensing Central Perturbations under Severe Finite-Source Effect

In the current strategy of microlensing planet searches focusing on high-magnification events, wide and close binaries pose important sources of contamination that imitates planetary signals. For the purpose of finding systematic differences, we compare the patterns of central perturbations induced by a planet and a binary companion under severe finite-source effect. We find that the most prominent difference shows up in the morphology of the edge features with negative excess that appear at the edge of the circle with its center located at the caustic center and a radius equivalent to the source radius. It is found that the feature induced a binary companion forms a complete annulus, while the feature induced by a planet appears as several arc segments. This difference provides a useful diagnostic for immediate iscrimination of a planet-induced perturbation from that induced by a binary companion, where the absence of a well-developed dip in the residual from the single-lensing light curve at both or either of the moments of the caustic center's entrance into and exit from the source star surface indicates that the perturbation is produced by a planetary companion. We find that that this difference is basically caused by the difference between the shapes of the central caustics induced by the two different types of companions.

astro-ph↗

arXiv2008

Planetary Lensing Signals of High-Magnification Events under Severe Finite-Source Effect

We investigate the effect of a finite source on the planetary-lensing signals of high-magnification events. From this, we find that the dependency of the finite-source effect on the caustic shape is weak and perturbations survive even when the source is substantially bigger than the caustic. Specifically, we find that perturbations with fractional magnification excess $\geq 5%$ survive when the source star is roughly 4 times bigger than the caustic. We also find characteristic features that commonly appear in the perturbation patterns of planetary lens systems affected by severe finite-source effect and thus can be used for the diagnosis of the existence of a companion. These features form in and around a circle with its center located at the caustic center and a radius corresponding to that of the source star. The light curve of an event where the source crosses these features will exhibit a distinctive signal that is characterized by short-duration perturbations of either positive or negative excess and a flat residual region between these short-duration perturbations.

astro-ph↗

arXiv2008

Microlensing Zone of Planets Detectable through the Channel of High-Magnification Events

A microlensing lensing zone refers to the range of planet-star separations where the probability of detecting a planetary signal is high. Its conventional definition as the range between $\sim 0.6$ and 1.6 Einstein radii of the primary lens is based on the criterion that a major caustic induced by a planet should be located within the Einstein ring of the primary. However, current planetary lensing searches focus on high-magnification events to detect perturbations induced by another caustic located always within the Einstein ring very close to the primary lens (stellar caustic) and thus a new definition of a lensing zone is needed. In this paper, we determine this lensing zone. By applying a criterion that detectable planets should produce signals $\geq 5%$, we find that the new lensing zone varies depending on the planet/star mass ratio unlike the fixed range of the classical zone regardless of the mass ratio. The lensing zone is equivalent to the classical zone for a planet with a planet/star mass ratio $q\sim 3\times 10^{-4}$ and it becomes wider for heavier planets. For a Jupiter-mass planet, the lensing zone ranges from 0.25 to 3.9 Einstein radii, corresponding to a physical range between $\sim 0.5$ AU and 7.4 AU for a typical Galactic event. The wider lensing zone of central perturbations for giant planets implies that the microlensing method provides an important tool to detect planetary systems composed of multiple ice-giant planets.

astro-ph↗

arXiv2008

Limits of Binaries That Can Be Characterized by Gravitational Microlensing

Due to the high efficiency of planet detections, current microlensing planet searches focus on high-magnification events. High-magnification events are sensitive to remote binary companions as well and thus a sample of wide-separation binaries are expected to be collected as a byproduct. In this paper, we show that characterizing binaries for a portion of this sample will be difficult due to the degeneracy of the binary-lensing parameters. This degeneracy arises because the perturbation induced by the binary companion is well approximated by the Chang-Refsdal lensing for binaries with separations greater than a certain limit. For binaries composed of equal mass lenses, we find that the lens binarity can be noticed up to the separations of $\sim 60$ times of the Einstein radius corresponding to the mass of each lens. Among these binaries, however, we find that the lensing parameters can be determined only for a portion of binaries with separations less than $\sim 20$ times of the Einstein radius.

astro-ph↗

arXiv2008

Microlensing Detections of Moons of Exoplanets

We investigate the characteristic of microlensing signals of Earth-like moons orbiting ice-giant planets. From this, we find that non-negligible satellite signals occur when the planet-moon separation is similar to or greater than the Einstein radius of the planet. We find that the satellite signal does not diminish with the increase of the planet-moon separation beyond the Einstein radius of the planet unlike the planetary signal which vanishes when the planet is located well beyond the Einstein radius of the star. We also find that the satellite signal tends to have the same sign as that of the planetary signal. These tendencies are caused by the lensing effect of the star on the moon in addition to the effect of the planet. We determine the range of satellite separations where the microlensing technique is optimized for the detections of moons. By setting an upper limit as the angle-average of the projected Hill radius and a lower limit as the half of the Einstein radius of the planet, we find that the microlensing method would be sensitive to moons with projected separations from the planet of $0.05 {\rm AU} \lesssim d_{\rm p} \lesssim 0.24 {\rm AU}$ for a Jupiter-mass planet, $0.03 {\rm AU}\lesssim d_{\rm p} \lesssim 0.17 {\rm AU}$ for a Saturn-mass planet, and $0.01 {\rm AU} \lesssim d_{\rm p} \lesssim 0.08 {\rm AU}$ for a Uranus-mass planet. We compare the characteristics of the moons to be detected by the microlensing and transit techniques

astro-ph↗

arXiv2008

A Characteristic Planetary Feature in Double-Peaked, High-Magnification Microlensing Events

A significant fraction of microlensing planets have been discovered in high-magnification events, and a significant fraction of these events exhibit a double-peak structure at their peak. However, very wide or very close binaries can also produce double-peaked high-magnification events, with the same gross properties as those produced by planets. Traditionally, distinguishing between these two interpretations has relied upon detailed modeling, which is both time-consuming and generally does not provide insight into the observable properties that allow discrimination between these two classes of models. We study the morphologies of these two classes of double-peaked high-magnification events, and identify a simple diagnostic that can be used to immediately distinguish between perturbations caused by planetary and binary companions, without detailed modeling. This diagnostic is based on the difference in the shape of the intra-peak region of the light curves. The shape is smooth and concave for binary lensing, while it tends to be either boxy or convex for planetary lensing. In planetary lensing this intra-peak morphology is due to the small, weak cusp of the planetary central caustic located between the two stronger cusps. We apply this diagnostic to five observed double-peaked high-magnification events to infer their underlying nature. A corollary of our study is that good coverage of the intra-peak region of double-peaked high-magnification events is likely to be important for their unique interpretation.

astro-ph↗

arXiv2008

Color-Shift Measurement in Microlensing-Induced Stellar Variation from Future Space-Based Surveys

If a microlensing event is caused by a star, the event can exhibit change in color due to the light from the lens. In the previous and current lensing surveys, the color shift could not be used to constrain the lens population because the blended light responsible for the color shift is mostly attributed to nearby background stars rather than the lens. However, events to be observed in future space-based surveys do not suffer from blending and thus the color information can be used to constrain lenses. In this paper, we demonstrate the usefulness of future surveys in measuring color shifts. By conducting simulation of galactic lensing events based on the specification of a proposed space-based lensing survey, we estimate that the shift in the color of $R-H$ will be measured at 5$σ$ level for $\sim 12%$ of events that occur on source stars with apparent magnitudes brighter than $J=22.5$. Color-shifted events tend to have high magnifications and the lenses will have brightnesses equivalent to those of source stars. The time scales of the color-shifted events tend to be longer than those without color shifts. From the mass distribution of lenses, we find that most of the color-shifted events will be produced by stellar lenses with spectral types down to mid M-type main sequence stars.

astro-ph↗

arXiv2008

Microlensing Search for Planets with Two Simultaneously Rising Suns

Among more than 200 extrasolar planet candidates discovered to date, there is no known planet orbiting around normal binary stars. In this paper, we demonstrate that microlensing is a technique that can detect such planets. Microlensing discoveries of these planets are possible because the planet and host binary stars produce perturbations at a common region around center of mass of the binary stars and thus the signatures of both planet and binary can be detected in the light curves of high-magnification microlensing events. The ranges of the planetary and binary separations of systems for optimal detection vary depending on the planet mass. For a Jupiter-mass planet, we find that high detection efficiency is expected for planets located in the range of $\sim$ 1 AU -- 5 AU from the binary stars which are separated by $\sim$ 0.15 AU -- 0.5 AU

astro-ph↗

arXiv2007

Microlensing Detections of Planets in Binary Stellar Systems

We demonstrate that microlensing can be used for detecting planets in binary stellar systems. This is possible because in the geometry of planetary binary systems where the planet orbits one of the binary component and the other binary star is located at a large distance, both planet and secondary companion produce perturbations at a common region around the planet-hosting binary star and thus the signatures of both planet and binary companion can be detected in the light curves of high-magnification lensing events. We find that identifying planets in binary systems is optimized when the secondary is located in a certain range which depends on the type of the planet. The proposed method can detect planets with masses down to one tenth of the Jupiter mass in binaries with separations <~ 100 AU. These ranges of planet mass and binary separation are not covered by other methods and thus microlensing would be able to make the planetary binary sample richer.

astro-ph↗

arXiv2007

Near-Field Microlensing from Wide-Field Surveys

We estimate the rate of near-field microlensing events expected from all-sky surveys and investigate the properties of these events. Under the assumption that all lenses are composed of stars, our estimation of the event rate ranges from Γ_{tot}~0.2 yr^{-1}$ for a survey with a magnitude limit of V_{lim}=12 to Γ_{tot}~20 yr^{-1} for a survey with V_{lim}=18. We find that the average distances to source stars and lenses vary considerably depending on the magnitude limit, while the dependencies of the average event time scale and lens-source transverse speed are weak and nearly negligible, respectively. We also find that the the average lens-source proper motion of events expected even from a survey with V_{lim}=18 would be <μ> >~ 40 mas yr^{-1}, implying that the source and lens of a significant fraction of near-field events could be resolved from high-resolution follow-up observations. From the investigation of the variation of the event characteristics depending on the position of the sky, we find that the average distances to source stars and lenses become shorter, the lens-source transverse speed increases, and the time scale becomes shorter as the the galactic latitude of the field increases. Due to the concentration of events near the galactic plane, we find that >~ 50 % of events would be detected in the field with b <= 20^\circ.

astro-ph↗

arXiv2007

Expansion of the Planet Detection Channels in Next-Generation Microlensing Surveys

We classify various types of planetary lensing signals and the channels of detecting them. We estimate the relative frequencies of planet detections through the individual channels with special emphasis on the new channels to be additionally provided by future lensing experiments that will survey wide fields continuously at high cadence by using very large-format imaging cameras. From this investigation, we find that the fraction of wide-separation planets that would be discovered through the new channels of detecting planetary signals as independent and repeating events would be substantial. We estimate that the fraction of planets detectable through the new channels would comprise ~15 -- 30% of all planets depending on the models of the planetary separation distribution and mass ratios of planets. Considering that a significant fraction of planets might exist in the form of free-floating planets, the frequency of planets to be detected through the new channel would be even higher. With the expansion of the channels of detecting planet, future lensing surveys will greatly expand the range of planets to be probed.

astro-ph↗